The present invention relates to an underroof for a sloping exterior roof and comprising a moisture absorbing layer.
It is well known to provide underroofs below sloping exterior roofs, such as tiled roofs, slate roofs, and roofs made from asbestos or cement slabs. The primary function of such underroofs is to prevent moisture, which penetrates the exterior roof through the inevitable gaps therein during heavy rain fall or the melting of snow drifts, from causing damage to the building. The present underroofs typically consist of reinforced plastic films which are both impermeable to water and have a high diffusion resistance. However, the use of such underroofs presents the problem that during the winter months water vapours tend to be condensed on the underside of the underroof in such amounts that severe damage may occur. In order to prevent such damage it is normally required that the loft be effectively ventilated. However, in practice this is not always possible.
If the loft has been converted into living quarters or if the ceiling located underneath the underroof extends parallel to the roof surface, the space between the underroof and such rooms or ceiling has to be thermally insulated.
In order to prevent the formation of condensate on the underroof it is normally necessary to provide a vapour barrier on the underside of the thermal insulation and to provide between the underroof and the insulation a space of a width of, e.g., 4 cm, which can be vented to the atmosphere.
In practice it has been found difficult to avoid the formation of condensate on the underroof because the vapour barrier on the underside of the insulation normally cannot be made sufficiently air tight to prevent humid air from migrating from the underlying rooms to the roof structure and to form condensate on the underroof.
Since the underroof normally hangs down between the rafters and tends to contact the insulation layer within small or large zones, the required ventilation space between the top side of the insulation and the underroof normally cannot be obtained.
In order to reduce the risk of moisture damage due to the formation of condensate on the underroof, the latter is often made from a material having a certain moisture absorbing capability and a low resistance to diffusion so that the absorbed condensate may diffuse through the underroof to the space between the exterior roof and the underroof, which space normally is effectively vented to the atmosphere. Even in cases where the resistance to diffusion of the underroof is low, the rate at which accumulated condensate diffuses to the underroof is relatively low. This is due to the fact that the thin films from which the underroof typically is made have an insignificant resistance to heat transfer compared to the total resistance to heat transfer exhibited by the roof construction.
In the well ventilated space between the top side of the underroof and the underside of the exterior roof the relative humidity will be the same as in the free air, which is typically close to 100% during the winter months. Since the temperature on the moist underside of the underroof will be equal to the temperature of the top side thereof, there will be no partial pressure difference to force the accumulated condensate through the underroof. Therefore, the condensate will remain in the underroof until, e.g., sunshine on the roof will force it back through the insulation to the vapour barrier on the underside of the insulation at which it will condensate and cause dripping from the ceiling. A low resistance to diffusion may also cause so-called summer condensation. Summer condensation arises when the roof coating has been wetted by rain and has absorbed moisture and the moisture under the influence of sunlight is forced into the underroof and the underlying heat insulation and is condensed on the relatively cold vapour barrier where it may cause moisture damage and lead to dripping from the ceiling.
It is also well known to prepare underroofs of fibrous materials, such underroofs having a top side coated with an essentially water-impermeable asphalt layer. The use of such an underroof may also lead to the formation of condensate on its underside and the condensate thus formed cannot penetrate the asphalt layer on the top surface of the underroof.
An ideal underroof should be impermeable to water flowing along the top side thereof, it should have a sufficiently high resistance to diffusion so that summer condensation is prevented, and it should be capable of absorbing condensate on its underside and allowing the condensate to migrate to its top side, where it should be capable of being evaporated.
If the underroof has these properties, it will not cause the above-mentioned condensation problems and the space between the heat insulation and the underroof can be either eliminated or used for increasing the thickness of the roofing.